API to access WKWebViewConfiguration

# Feature Request - API to access WKWebViewConfiguration

## Motivation Behind Feature

Our team is upgrading from Cordova 5 to 6 and migrating from `UIWebView` to `WKWebView`. Our app used to use `NSURLProtocol` subclasses to handle certain requests from the UIWebView. This does not work with `WKWebView`. The new approach is to use `WKURLSchemeHandler` instances to handle requests made with a custom scheme. In order to do this, we need to register our scheme handlers with the `WKWebViewConfiguration` for the WebView. Right now, there does not seem to be a way to do this with public API. We also need to set the `websiteDataStore` on the `WKWebViewConfiguration` in order to set cookies for the WebView.

## Feature Description

One option: Make `CDVWebViewEngine` a public class that class that can be extended. That way we can override `createConfigurationFromSettings:` and access `WKWebViewConfiguration`

Also open to any other option that provides access to `WKWebViewConfiguration`

## Alternatives or Workarounds

Our current workaround is to use a `post_install` action in CocoaPods moves `CDVWebViewEngine.h` and `CDVWebViewUIDelegate.h` from private to public, then extending `CDVWebViewEngine`

CC: @lukemelia

Start by reading CDVWebViewEngine.h and CDVWebViewUIDelegate.h, focusing on createConfigurationFromSettings: and how the WKWebViewConfiguration is created. Compare the current private-header workaround with possible public API designs; done means a decided, supported way to configure WKURLSchemeHandler and websiteDataStore without moving private headers.

Written by the indexing model from the issue text.